The HTML5Backend should determine draggable from canDrag

[anuraags]

Is your feature request related to a problem? Please describe.

Currently there is a canDrag predicate that can be assigned to a drag source in react-dnd. However, the HTML5Backend currently sets "draggable" to true for a drag source regardless what this predicate returns. It should actually set the draggable property in this line:

react-dnd/packages/backend-html5/src/HTML5BackendImpl.ts

Line 144 in ed963cd

node.setAttribute('draggable', 'true')

based on canDrag from the monitor or options.

This can allow developers to prevent bugs such as this one:
#438

by setting canDrag based on a focus predicate.

Describe the solution you'd like

That when canDrag is false for a drag source, the draggable attribute for that drag source ref is set to false.